The immunocompetence of patients with astrocytomas and glioblastomas is being studied using the in vitro lymphocyte blast transformation to mitogens PHA and CON A. Plasma from 40 patients with glioblastoma multiforme have been studied, with 90% of these patients demonstrating plasma inhibition of autologous and normal lymphocytes. Inhibition of autologous lymphocytes was always greater than inhibition of normal lymphocytes. This same inhibition was demonstrated in only 1 of 10 patients with Grade I or II astrocytomas. The one patient demonstrating inhibition was terminal. The degree of plasma suppression continues to correlate with the clinical condition of the patient; i.e. the greater the degree of plasma inhibition, the worse is the clinical condition of the patient. The inhibitory capacity of glioblastoma plasma is dependent on the concentrations of plasma and gives a linear dose response curve when the blastogenic response is plotted against the concentration of serum added. This study indicates that suppressed autologous lymphocyte function is related to the presence of the plasma inhibitory factor.